Blender 2.8+ set pass index for object?
You can find it in the Properties Editor, in the Object tab:



You might also find it useful to use a very simple Python script to set different object indices for multiple objects(all selected in this case) at once:

import bpy

some_starting_number = 15

for i,o in enumerate(bpy.context.selected_objects):
    o.pass_index = i + some_starting_number


(You would copy this into a new text block in the Text Editor and hit Run Script with some objects selected)